Vehicle repair shops often find it necessary to remove the engine or transmission of a vehicle in order to effect necessary repairs. Obviously, the engine and transmission are too heavy for one or more repair men to manually remove from the vehicle, at least without an unnecessary risk of injury to themselves. Accordingly, mobile hydraulic jacks have been used for a number of years to facilitate removal and reinstallation of vehicle engines and transmissions. Typically, the vehicle is raised on a conventional hoist, and then the mobile hydraulic jack is moved beneath the raised vehicle, the hydraulic cylinder of the jack is extended to raise a load supporting frame into engagement with the engine or transmission, and then the engine or transmission can be detached from the vehicle and lowered out of the vehicle by the jack.
In conventional jacks of this type, extension of the piston of the hydraulic cylinder in order to raise the load supporting frame is normally effected by repeatedly operating a manual lever which drives a manual pump which in turn forces hydraulic fluid from a reservoir into the hydraulic cylinder. However, manually pumping the hydraulic fluid into the hydraulic cylinder is very tiring, and relatively time consuming.
One commercially available jack can use compressed air to effect a first stage of upward movement of the load supporting frame, but then the remainder of the upward movement of the load supporting frame must be effected in the traditional manner by repeatedly manually operating a lever coupled to a manual pump which forces hydraulic fluid into the hydraulic cylinder. In this known jack, the pneumatic and hydraulic capabilities are functionally separate, the hydraulic operation still being entirely manually controlled, with the traditional disadvantages.
An object of the present invention is therefore to provide a hydraulic jack which has no manually operated pump, and in particular in which the hydraulic operation is powered entirely by compressed air.
A further object is to provide such a jack in which all hydraulic and pneumatic valves are operated by a single foot pedal.
A further object is to provide such a jack in which the hydraulic valve arrangement will prevent downward movement of the load supporting frame when the operator has selected upward movement, so that a sudden and unexpected loss of pressurized air to the jack will not result in a sudden and unexpected downward movement of the load supporting frame or the associated risk of operator injury.
A further object is to provide such a jack which is durable and will provide a long and reliable operational lifetime with little or no maintenance